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Mon Feb 26, 2018, 11:07 PM

The Conversion of Cellulosic Biomass Into Aromatic Compounds.

Most of the chemicals utilized in the preparation of polymers are currently derived from petroleum, and to the extent they are degraded either by combustion or other means, the represent a climate risk.

Since we have, in our times, spectacularly failed to address climate change, offering in lieu of things that actually work truckloads of wishful thinking (solar and wind) future generations will need to clean up our mess, the biggest mess being the planetary atmosphere and the oceans with which they are in a shifting equilibrium.

Although plastics represent a huge environmental problem, the problem would be mitigated to the extent that they were obtained from air rather than from petroleum, since in the former case, they would represent sequestered carbon.

Some years back, there was a lot of talk about converting cellulosic materials into ethanol via fermentation schemes of various types. All of these efforts have more or less commercially failed, probably because, among other things, they were water and energy intensive.

However, the chemical dehydration of cellulosic materials, generally with acid and heat, is known to produce compounds in a class known as furans, five membered unsaturated rings, generally with one or two single carbon side chains. (Historically almost all the furan in the world was made from oat hulls, until petrochemicals replaced them via a route from butadiene obtained from dangerous fossil fuels.)

Here, for example is the structure of dimethylfuran:

A problem with biomass to chemicals conversion has been, however, the low production of aromatic compounds like benzene, toluene and the xylenes, one of which p-xylene, aka as 1,4-dimethylbenzene is an important precursor to common plastics like PET, polyethylene terephthalate, a polymer used in bottles and in clothing. Xylenes are also important constituents of cleaners, fuels, paints and varnishes, especially those requiring special properties such as in works of art.

It is thus with interest I came across the following paper published by Chinese scientists in the scientific literature: One-Step Conversion of Biomass-Derived Furanics into Aromatics by Brønsted Acid Ionic Liquids at Room Temperature (Zhang et al, ACS Sustainable Chem. Eng., 2018, 6 (2), pp 2541–2551)

The introduction covers what I just said.

Aromatics are elementary commodity products from petroleum resources. For example, p-xylene (PX) is a fundamental aromatic hydrocarbon and serves as the feedstock for the production of polyethylene terephthalate (PET), polybutylene terephthalate (PBT), coatings, dyes, and so on.1−3 With a research octane number of 127 and low toxicity, PX is regarded as an excellent octane booster while the price has limited its application.4 Since the last century, fossil resources have constituted the main feedstocks for the production of most fuels, chemicals, and materials, but the environmental concerns together with diminishing fossil reserves result in a global challenge. Efficient processes that enable the production of valuable products from renewable feedstocks with high yields must be developed to reduce global warming while satisfying the growing energy demands...

The authors express their goal and what they have accomplished a little further on:

one-step synthetic route at mild conditions with high selectivity would be a significant advance in the conversion of furanics into aromatics. Herein, we report a novel process to efficiently obtain PX and 2,5-methylbenzoic acid (2,5-DMBA) by acidic ionic liquids from biobased DMF and acrylic acid which can be produced by oxidative dehydration of the side-product from biodiesel production (i.e., glycerol).34,35 The reaction including Diels−Alder cycloaddition, dehydration, and decarboxylation could be accomplished conveniently at room temperature and atmospheric pressure as shown in Scheme 1c. The predominant success of this efficient conversion relies on the unique properties of ionic liquids (ILs) used as solvents and acidic catalysts to suppress the retro Diels−Alder reactions and side-reactions. For further understanding of the reaction mechanism, isotopic labeling and computational study were used. We also highlighted the generality of this catalytic system with a series of related furanic compounds and dienophiles; then, moderate aromatic yields were obtained, and the influence of different substituents of the reactants are also described.

Reference 35, on dehydrating and oxidizing glycerol to give acrylic acid is this one from the same journal: Highly Selective Production of Acrylic Acid from Glycerol via Two Steps Using Au/CeO2 Catalysts

The "ionic liquids" are salts, usually with one or two organic species, that are liquid at or near room temperature. Most of those used in the paper are of the alkylimidazole type, albeit mostly with inorganic counterions like sulfates or phosphates.

Some pictures from the paper:

The picture shows a tree as the source, but this is unnecessary, straw and things like corn cobs would work quite as well, if not better. Wood, by the way, is a source of aromatic compounds, from the lignin they contain in addition to cellulose, but in general they are phenolic, having one or more acidic -OH moieties on the aromatic ring.

Here the authors compare their work, (c), with those of previous authors working on the same idea, furans to aromatics:

Here is some graphics showing the yields and selectivity under several conditions:

The caption:

Figure 1. (a) Conversion of DMF and yield and selectivity of aromatics as a function of time at 25 °C. (b) Effect of temperature on DMF conversion and aromatic yield. (c) Effect of temperature on aromatic selectivity. Reaction conditions: 1 mmol of DMF, 6.9 mmol of acrylic acid, 2 mmol of [Bmim]HSO4, (b, c) 60 min (10, 25 °C), 30 min (40, 55, 70 °C).

PX is paraxylene, 2,5 DMBA is 2,5 dimethyl benzoic acid.

Reaction parameters:

The caption:

Figure 2. (a) Kinetics of the reaction between DMF and acrylic acid at different temperatures. Reaction conditions: 1 mmol of DMF, 6.9 mmol of acrylic acid, 2 mmol of [Bmim]HSO4. (b) Arrhenius plot for the conversion of DMF catalyzed by [Bmim]HSO4 from 10 to 70 °C.

Reaction mechanisms leading to the two main aromatic products:

Note the role of the inorganic anions.

The free energy diagram associated with this mechanism:

The authors thus conclude:

In summary, the one-step conversion of biobased furanics into aromatics via a synthetic route including Diels−Alder, dehydration, and decarboxylation reactions can be efficiently catalyzed by acidic ILs at mild conditions. [Bmim]HSO4 gave high yield of PX and 2,5-DMBA from DMF and acrylic acid with up to 89% aromatic selectivity in a single step at room temperature. The reaction mechanism supported by computational simulation and isotopic tracing was studied, and the energy barriers of every elementary step were presented. With application of [BSO3HMIm]HSO4 to the reactions using different dienes and dienophiles, moderate yields of various aromatics were obtained, which suggested the great potential to obtain excellent yield of renewable aromatics by tuning the structure and properties of ILs, particularly the acidity. It was also proven that the electron-donating methyl groups on the furan ring could significantly benefit the dehydration and decarboxylation processes.

Interesting I think, esoteric but interesting.

Have a pleasant Tuesday tomorrow.

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